Buoyancy system for an underwater device and associated methods for operating the same

ABSTRACT

A buoyancy system includes a chamber having a volume associated therewith, and bladders within the volume of the chamber. Each bladder contains a clathrate mixture in a liquid state. The chamber includes an opening to allow surrounding water to circulate within the volume and contact the bladders. As the chamber is submerged in the surrounding water, the bladders expand based on the clathrate mixture changing from the liquid state to a solid state. This changes buoyancy by allowing less water to circulate within the volume of the chamber.

FIELD OF THE INVENTION

The present invention relates to the field of underwater devices, andmore particularly, to a buoyancy system for controlling buoyancy of anunderwater device.

BACKGROUND OF THE INVENTION

An underwater glider is a type of underwater device that collectssubsurface data in an observation region. The underwater glider istypically a torpedo shaped, winged device that moves through the waterin a saw-tooth sampling pattern by changing its buoyancy. The underwaterglider is neutrally buoyant, and typically includes a buoyancy system inits nose section.

The buoyancy system may be based on a displacement piston. To diver thedisplacement piston moves water into nose section of the underwaterdevice. This makes the underwater glider's nose heavy. To ascend, wateris pushed out of the nose section by the displacement piston. This makesthe underwater glider's nose lighter.

Even in view of the advances made in buoyancy systems, there is still aneed to improve such systems. For example, U.S. Pat. No. 6,131,531discloses a selectively deformable buoyancy system. The buoyancy systemincludes a housing having walls defining an interior, sealable cavity.Changing the volume of the cavity controls buoyancy. The cavity has anoriginal volume when the walls are maintained at or above a preselectedtemperature. The walls are deformed at temperatures below thepreselected temperature to define a volume less than the originalvolume. The housing returns to the original volume when the temperatureof the walls is raised above the preselected temperature.

Composite materials may be used as part of a buoyancy system, asdisclosed in U.S. Pat. No. 4,482,590. In particular, implosion resistantmacrospheres for use in buoyancy systems may be fabricated fromsynthetic foams, preferably from synthetic thermosetting polymericresins. The implosion resistant macrospheres are primarily used inbuoyancy devices at sea depths in excess of 4,500 feet.

SUMMARY OF THE INVENTION

In view of the foregoing background, it is therefore an object of thepresent invention to provide an improved buoyancy system for controllingbuoyancy of an underwater device.

This and other objects, advantages and features in accordance with thepresent invention are provided by a buoyancy system comprising a chamberhaving a volume associated therewith, and a plurality of bladders withinthe volume of the chamber. Each bladder may contain a clathrate mixturein a liquid state. The chamber may include at least one opening to allowsurrounding water to circulate within the volume. When the chamber issubmerged in increasingly frigid surrounding water, the plurality ofbladders may expand based on the clathrate mixture changing from theliquid state to a solid state. This thereby increases buoyancy byallowing less water to circulate within the volume of the chamber.

Similarly, when the chamber ascends in the increasingly warm surroundingwater, the plurality of bladders may contract based on the clathratemixture changing from the solid state to the liquid state. This therebydecreases buoyancy by allowing more water to circulate within the volumeof the chamber.

Each bladder may comprise a water-tight enclosure so that the clathratemixture therein does not directly contact the water. The clathratemixture may comprise water and a clathrating agent. Each bladder maymaintain a predetermined pressure on the clathrate mixture so that theclathrating agent does not vaporize when the clathrate mixture is in theliquid state. Vaporization of the clathrating agents would make anunderwater device with such a buoyancy system permanently buoyant. Themaintained minimum pressure thus depends on the clathrating agent, sinceeach clathrating agent has a unique dissolution pressure.

Each bladder may comprise an elastic enclosure that expands as theclathrate mixture changes to the solid state. The elastic enclosure maycomprise a thermally conductive material. The thermally conductivematerial advantageously allows the temperature of the surrounding waterto be efficiently transferred to the clathrate mixture. As the watertemperature cools, the clathrate mixture decreases density when itbegins to freeze. The clathrate mixture expands as it freezes, similarto an ice cube that floats.

Once the clathrate mixture reaches a depth in the water where it canbegin forming ice, each bladder expands as a result of the volumeincrease of the ice. This volume increase, multiplied for the totalnumber of bladders, causes water to be forced out of the chamber. Thisdecreases the overall mass while displacing the same volume of water. Asa result, the buoyancy changes. The same concept applies in reverse asthe ice melts. The bladders will shrink and the buoyancy system willweigh more as more water is allowed to enter the chamber, and itsbuoyancy will change again.

The buoyancy system may further comprise a respective spacer coupledbetween adjacent bladders so that the bladders are spaced apart from oneanother within the volume of the chamber. This advantageously helps withthe transfer of heat from the water to the clathrate mixture since thewater will surround each bladder, as compared to partially surroundingthe bladders when they are bunched up against one another. Moreover,each bladder may be spherically shaped to provide a greater surface areafor the water to contact, thereby improving heat transfer.

The bladders may form a three-dimensional array of bladders. Thebuoyancy system may further comprise a water permeable enclosuresurrounding the plurality of bladders within the volume of the chamber.The water permeable enclosure advantageously prevents anyone of thebladders from escaping the chamber.

Another aspect of the present invention is directed to an underwaterdevice comprising a housing, and a buoyancy system carried by thehousing. The buoyancy system may be as defined above. The housing andthe buoyancy system may be configured so that the underwater device isan underwater glider or a sonar buoy, for example.

Yet another aspect of the present invention is directed to a method forchanging buoyancy of an underwater device comprising a buoyancy systemas described above. The method may comprise placing the underwaterdevice in the water, and submerging the underwater device based on thesurrounding water entering the at least one opening within the chamberand contacting the plurality of bladders. The method may furthercomprise expanding the plurality of bladders based on the clathratemixture changing from the liquid state to a solid state so that lesswater is circulated within the volume of the chamber, thereby changingthe buoyancy of the underwater device. The method may further comprisecontracting the plurality of bladders after having been expanded, withthe contracting being based on the clathrate mixture changing from thesolid state back to the liquid state so that more water is circulatedwithin the volume of the chamber, thereby changing the buoyancy of theunderwater device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of an underwater glider with abuoyancy system in accordance with the present invention.

FIG. 2 is a schematic block diagram of a sonar buoy with a buoyancysystem in accordance with the present invention.

FIG. 3 is a block diagram of a buoyancy system, wherein each bladdertherein comprises a clathrate in a liquid state in accordance with thepresent invention.

FIG. 4 is a block diagram of a buoyancy system, wherein each bladdertherein comprises a clathrate in a solid state in accordance with thepresent invention.

FIG. 5 is a block diagram of a buoyancy system, wherein the bladderstherein form a three-dimensional array of bladders in accordance withthe present invention.

FIG. 6 is block diagram of a buoyancy system, wherein a water permeableenclosure surrounds the bladders in accordance with the presentinvention.

FIG. 7 is a flow chart for a method for changing buoyancy of anunderwater device in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. Likenumbers refer to like elements throughout.

Referring initially to FIG. 1, an underwater device 10 comprises ahousing 12, and a buoyancy system 20 carried by the housing. Theunderwater device 10 is illustrated as an autonomous underwater gliderthat may be used to collect subsurface data in an observation region.The underwater glider is neutrally buoyant, and travels through thewater in a saw-tooth-sampling pattern 22 by using the buoyancy system 20to change its buoyancy. Although the underwater device 10 is illustratedas an autonomous underwater glider, the buoyancy system 20 is readilyapplicable to other types of underwater devices, such as a sonar buoy,for example, as illustrated in FIG. 2.

As will be discussed in greater detail below, the buoyancy system 20changes buoyancy of the underwater device 10 based on the use of aplurality of bladders 30, where each bladder contains a clathratemixture. The plurality of bladders 30 may also be referred to as ablister pack. The clathrate mixture comprises water and a clathratingagent, as will be discussed in greater detail below.

The bladders 30 are in contact with the water, and expand or contractbased on the effects of the water's temperature on the clathratemixture. The expansion or contraction of the bladders 30 affects thebuoyancy of the underwater device 10.

The clathrate mixture changes from a liquid state to a sold state as theunderwater device 10 submerges, and from the sold state back to theliquid state as the underwater device 10 rises. When the clathratemixture is in the solid state, the mixture may also be referred to as aclathrate hydrate. Transition between the liquid and solid states isbased on the temperature of the water surrounding the bladders 30. Thesharpness of the saw-tooth-sampling pattern 22 depends on the rate offusing the clathrate hydrate (bottom saw-tooth-sampling pattern) and onthe rate of melting the clathrate hydrate (top saw-tooth-samplingpattern).

Referring now to FIGS. 3 and 4, the buoyancy system 20 comprises achamber 24 having a volume associated therewith, and the bladders 30 arewithin the volume of the chamber. The chamber 24 includes an opening 40facing the nose of the underwater device 10, and allows water to flowinto the chamber 24 and contact the bladders 30. The chamber 24 mayfurther include another opening 42 facing the rear of the underwaterdevice 10 so that the water exits the chamber 24 as fresh water enters.Buoyancy of the underwater device 10 is based on the amount of waterwithin the chamber 22, which is based on how much of the clathratemixture within the bladders 30 are in a liquid and/or solid state.

As readily appreciated by those skilled in the art, the clathratemixture comprises water and a clathrating agent. The clathrating agentmay include, but are not limited to, methane or propane, for example.The clathrate agent is not limited to a single type clathrating agent.The bladder 30 may include more than one type of clathrating agent. Whenthe clathrate is in the solid state, the clathrate mixture is alsoreferred to as a clathrate hydrate. Clathrate hydrates are crystallinecompounds defined by the inclusion of a guest molecule within a hydrogenbonded water lattice. Gas hydrates are a subset of clathrate hydrateswherein the guest molecule is a gas at or near ambient temperatures andpressures. Such gasses include methane, propane, carbon dioxide,hydrogen, for example; although not all the gas hydrates are suitablefor buoyancy modulation when their solidified state is denser than theliquid state (as is the case for carbon dioxide hydrate).

The clathrate mixture changes density when it freezes. As bestillustrated in FIG. 3, the clathrate mixture is in a liquid state sothat the bladders 30 are at a normal size. The bladders 30 are separatedfrom one another when the clathrate mixture is in the liquid state. Whenthe clathrate mixture freezes, the bladders 30 expand, as bestillustrated in FIG. 4. Instead of being separated from one another, thebladders 30 now are closer with one another when in the solid state,possibly in contact with one another.

Each bladder 30 comprises a water-tight enclosure so that the clathratemixture therein does not directly contact the water. The water-tightenclosure could be a variety of plastics or rubber compounds that areelastic enough to accommodate the expanding clathrate as it freezes, butrigid enough to prevent any leaks between the ambient water and theclathrates.

Each bladder 30 maintains a predetermined pressure on the clathratemixture so that the clathrating agent does not vaporize when theclathrate mixture is in the liquid state. For example, if theclathrating agent is propane, then the bladder maintains a pressure ofat least 150 psi so that the propane does not vaporize when theunderwater device 10 is at the surface of the water. Vaporization of theclathrating agent would make the underwater device 10 permanentlybuoyant. The maintained minimum pressure thus depends on the clathratingagent, since each clathrating agent has a unique dissolution pressure.

As noted above, each bladder 30 comprises an elastic enclosure thatexpands as the clathrate mixture changes to the solid state. The elasticenclosure also comprises a thermally conductive material. The thermallyconductive material advantageously allows the temperature of the waterto be efficiently transferred to the clathrate mixture. As the watertemperature cools, the clathrate mixture fuses into clathrate hydratewhich decreases in density as the mass expands. This is similar to anice cube that floats. As readily appreciated by those skilled in theart, some clathrate hydrates become more dense than their respectiveclathrate mixture states, and consequently, these clathrating agents arenot appropriate for use with a buoyancy system 20. Once the clathratemixture reaches a depth in the water where it can begin forming ice,each bladder 30 expands as a result of the volume increase of the ice.

When the clathrating agent is propane, for example, the propane fuseswith water at about 6 degrees Celsius. This volume increase, multipliedfor the total number of bladders 30, causes water to be forced out ofthe chamber 24. This decreases the overall mass while displacing thesame change in volume of water. As a result, the buoyancy increases. Thesame concept applies in reverse as the clathrate hydrate melts. Thebladders 30 will shrink and the buoyancy system 20 will weigh more asmore water is allowed to enter the chamber 24, and its buoyancy willchange again.

The size and number of bladders 30 within the chamber 24 will varydepending on the size or volume of the chamber, as well as the intendedapplication of the underwater device 10. There needs to be enoughbladders 30 to provide enough clathrate mixtures within the chamber 24to effect a density change in the underwater device 10 to reverse itsbuoyancy. This would also depend on the volume and weight of theunderwater device 10, and the desired climb or dive rates that may berequired.

For illustrative purposes, the size of each bladder 30 may be within arange of about 1/16 to 2 inches, for example. The size of the chamber 24is typically about 10 to 20% of the total volume of the underwaterdevice 10. For an underwater device 10 that is about 25 cubic feet involume, the chamber 24 would have a volume of about 2.5 to 5 cubic feet.There needs to be a sufficient number of bladders 20 to displace waterfrom the chamber 24 so that there is an effect on buoyancy of theunderwater device 10.

The number of bladders 30 within the chamber 24 may also compensate forfailure of a certain number of bladders 30 that is expected over time.Consequently, additional bladders 30 may be included within the chamber24 so that buoyancy can still be controlled even with the loss of aportion of the bladders 30.

As illustrated in FIG. 3, the buoyancy system 20 may further comprise arespective spacer 32 coupled between adjacent bladders 30 so that thebladders are spaced apart from one another within the volume of thechamber 24. This advantageously helps with the transfer of heat from thewater to the clathrates since the water will surround each bladder, ascompared to partially surrounding the bladders when they are bunched upagainst one another. Moreover, each bladder 30 may be spherically shapedto provide a greater surface area for the water to contact.

Referring now to FIG. 5, the bladders 30 may be coupled together so thatthey form a three-dimensional array of bladders. This resembles atomiccrystal structures to maximize packing of the bladders 30 within thechamber 24. The buoyancy system 20 may further comprise a waterpermeable enclosure 50 surrounding the plurality of bladders within thevolume of the chamber, as illustrated in FIG. 6. The water permeableenclosure 50 advantageously prevents anyone of the bladders fromescaping the chamber 24.

Another aspect of the invention is directed to a method for changingbuoyancy of an underwater device 10 comprising a buoyancy system 20 asdescribed above. Referring now to FIG. 7, from the start (Block 70), themethod comprises placing the underwater device 10 in the water at Block72. The water needs to have a temperature at depth that is cold enoughto freeze the clathrate mixture within the bladders 30. The water needsto have a temperature at the surface that is warm enough to melt theclathrate mixture within the bladders 30. The underwater device 10submerges and sinks from the surface based on the surrounding waterflooding the chamber 24 and engulfing the bladders 30 at Block 74.

The bladders 30 then expand at Block 76 as the clathrate mixture changesfrom a liquid state to a solid state so that less water is circulatedwithin the volume of the chamber 24. This increases the buoyancy of theunderwater device 10. As a result, the underwater device 10 floatstoward the surface of the water at Block 78. The bladders 30 contactwarmer water causing the clathrate mixture in the solid state to meltback into the liquid state.

The cycle of descending and ascending is continuously repeated in Blocks74, 76 and 78. To end this cycle, the bladders 30 may rupture over timedue to environmental causes at Block 80. Similarly, the bladders 30 mayfail when their elastic properties become brittle over time at Block 82.Yet another option for ending this cycle is to scuttle the underwaterdevice 10 by rupturing the bladders 30 on purpose at Block 84. Themethod ends at Block 86.

The blister pack approach has a significant lifecycle advantage over apiston cylinder device or a single large bladder device. Both thesedevices are disclosed in U.S. patent application Ser. No. 12/017,966,which is incorporated herein by reference in its entirety and isassigned to the current assignee of the present invention.

In the illustrated buoyancy system 20, the failure of any single bladderwill have little effect on the overall performance of a buoyancy cycle.It would take a large number of bladder failures to terminate thebuoyancy cycle. By eliminating single points of failure, an underwaterdevice 10 including a plurality of bladders 30 will have a longerendurance. Its performance eventually will gradually diminish asindividual bladder failures accumulate over time, as opposed to thecatastrophic failure that would occur with a piston cylinder device orwith a large bladder device. The blister pack bladder approach has asignificant production advantage over the piston cylinder device or thelarge bladder device since the bladders 30 can be more easily massproduced.

Many modifications and other embodiments of the invention will come tothe mind of one skilled in the art having the benefit of the teachingspresented in the foregoing descriptions and the associated drawings.Therefore, it is understood that the invention is not to be limited tothe specific embodiments disclosed, and that modifications andembodiments are intended to be included within the scope of the appendedclaims.

1. A buoyancy system comprising: a chamber having a volume associatedtherewith; and a plurality of bladders within the volume of saidchamber, each bladder containing a clathrate mixture in a liquid state;said chamber including at least one opening to allow surrounding waterto circulate within the volume and contact said plurality of bladders,and as said chamber is submerged in the surrounding water said pluralityof bladders expand based on the clathrate mixture changing from theliquid state to a solid state, thereby changing buoyancy by allowingless water to circulate within the volume of said chamber.
 2. Thebuoyancy system according to claim 1, wherein the clathrate mixturecomprises water and a clathrating agent; and wherein the clathratingagent comprises at least one of methane, floro-methane, propane,floro-propane and hydrogen.
 3. The buoyancy system according to claim 1,wherein each bladder comprises a water-tight enclosure so that theclathrate mixture therein does not directly contact the water.
 4. Thebuoyancy system according to claim 1, wherein each bladder comprises anelastic enclosure that expands as the clathrate mixture changes to thesolid state.
 5. The buoyancy system according to claim 4, wherein theelastic enclosure comprises a thermally conductive material.
 6. Thebuoyancy system according to claim 1, wherein the clathrate mixturecomprises water and a clathrating agent, and each bladder maintains apredetermined pressure on the clathrate mixture so that the clathratingagent does not vaporize when the clathrate mixture is in the liquidstate.
 7. The buoyancy system according to claim 1, further comprising awater permeable enclosure surrounding said plurality of bladders withinthe volume of said chamber.
 8. The buoyancy system according to claim 1,wherein each bladder is spherically shaped.
 9. The buoyancy systemaccording to claim 1, further comprising a respective spacer coupledbetween adjacent bladders so that said bladders are spaced apart fromone another within the volume of said chamber.
 10. The buoyancy systemaccording to claim 1, wherein said plurality of bladders form athree-dimensional array of bladders.
 11. An underwater devicecomprising: a housing; and a buoyancy system carried by said housing,and comprising a chamber having a volume associated therewith, and aplurality of bladders within the volume of said chamber, each bladdercontaining a clathrate mixture in a liquid state, said chamber includingat least one opening to allow surrounding water to circulate within thevolume and contact said plurality of bladders, and as said chamber issubmerged in the surrounding water said plurality of bladders expandbased on the clathrate mixture changing from the liquid state to a solidstate, thereby changing buoyancy of the underwater device by allowingless water to circulate within the volume of said chamber.
 12. Theunderwater device according to claim 11, wherein the clathrate mixturecomprises water and a clathrating agent; and wherein the clathratingagent comprises at least one of methane, floro-methane, propane,floro-propane and hydrogen.
 13. The underwater device according to claim11, wherein each bladder comprises a water-tight enclosure so that theclathrate mixture therein does not directly contact the water.
 14. Theunderwater device according to claim 11, wherein each bladder comprisesan elastic enclosure that expands as the clathrate mixture changes tothe solid state.
 15. The underwater device according to claim 14,wherein the elastic enclosure comprises a thermally conductive material.16. The underwater device according to claim 11, wherein the clathratemixture comprises water and a clathrating agent, and each bladdermaintains a predetermined pressure on the clathrate mixture so that theclathrating agent does not vaporize when the clathrate mixture is in theliquid state.
 17. The underwater device according to claim 11, furthercomprising a water permeable enclosure surrounding said plurality ofbladders within the volume of said chamber.
 18. The underwater deviceaccording to claim 11, wherein each bladder is spherically shaped. 19.The underwater device according to claim 11, further comprising arespective spacer coupled between adjacent bladders so that saidbladders are spaced apart from one another within the volume of saidchamber.
 20. The underwater device according to claim 11, wherein saidhousing and said buoyancy system are configured so that the underwaterdevice is an underwater glider.
 21. The underwater device according toclaim 11, wherein said housing and said buoyancy system are configuredso that the underwater device is a sonar buoy.
 22. A method for changingbuoyancy of an underwater device comprising a buoyancy system, thebuoyancy system comprising a chamber having a volume associatedtherewith, and includes at least one opening to allow water to circulatewithin the volume, and a plurality of bladders within the volume of thechamber, with each bladder containing a clathrate mixture in a liquidstate, the method comprising: placing the underwater device in thewater; submerging the underwater device based on the surrounding waterentering the at least one opening within the chamber and contacting theplurality of bladders; and expanding the plurality of bladders based onthe clathrate mixture changing from the liquid state to a solid state sothat less water is circulated within the volume of the chamber, therebychanging the buoyancy of the underwater device.
 23. The method accordingto claim 22, further comprising contracting the plurality of bladdersafter having been expanded, the contracting based on the clathratemixture changing from the solid state back to the liquid state so thatmore water is circulated within the volume of the chamber, therebychanging the buoyancy of the underwater device.
 24. The method accordingto claim 22, wherein the clathrate mixture comprises water and aclathrating agent; and wherein the clathrating agent comprises at leastone of methane, floro-methane, propane, floro-propane, and hydrogen. 25.The method according to claim 22, wherein each bladder comprises awater-tight enclosure so that the clathrate mixture therein does notdirectly contact the water.
 26. The method according to claim 22,wherein each bladder comprises an elastic enclosure that expands as theclathrate changes to the solid state.
 27. The method according to claim26, wherein the elastic enclosure comprises a thermally conductivematerial.
 28. The method according to claim 22, wherein each bladdermaintains the clathrate mixture pressure above the vaporization pressureof the clathrating agent.
 29. The method according to claim 22, whereinthe buoyancy system further comprises a water permeable enclosuresurrounding the plurality of bladders within the volume of the chamber.30. The method according to claim 22, wherein each bladder isspherically shaped.
 31. The method according to claim 22, wherein thebuoyancy system further comprises a respective spacer coupled betweenadjacent bladders so that the bladders are spaced apart from one anotherwithin the volume of the chamber.
 32. The method according to claim 22,wherein the housing and the buoyancy system are configured so that theunderwater device is at least one of an underwater glider and a sonarbuoy.